This invention relates to ground based radar antenna systems for use in tracking targets, and more particularly, to a new and improved dual beam ground based radar antenna system and method having a pair of feed horns and a reflector for determining the height of a tracked target.
In the field of electronic tracking devices, the use of ground based radar antennas have long been recognized as an effective way to determine the range and bearing of a tracked target. Such ground based radar antenna installations are commonly found along the boarders and coastlines and on the military reservations of modern industrialized nations.
Because of the continued military and industrial development of high altitude supersonic aircraft, radar antenna installations of the past are faced with a continuing difficulty of determining the altitude and three-dimensional position of an aircraft. In particular, when applied from a ground based radar installation, the basic measurement required for determining height is the elevation angle of the aircraft. In order to properly track the aircraft, the height of the target must be known. Generally, once the elevation angle of the target is known, the height or altitude of the target is derived from the elevation angle by trigonometric formulas.
General concepts of the radar technology may be gleaned from a handbook entitled INTRODUCTION TO RADAR SYSTEMS authored by Merrill I. Skolnik, copyrighted 1962 edition by McGraw-Hill Book Company. Two of the basic beam patterns that exist are the pencil beam and the fan beam. The pencil beam may be generated with a metallic reflector surface shaped in the form of a paraboloid of revolution with the electromagnetic energy fed from a point source placed at the focus. Although a narrow beam can search a large sector or even a hemisphere, it is not always desirable because operational requirements place a restriction on the maximum scan time. The maximum scan time is defined as the time for the pencil beam to return to the same point in space. Therefore, the radar beam cannot dwell too long in any one radar location. This is especially true if there is a large number of locations to be searched. The number of locations to be searched can be materially reduced if the narrow pencil beam radar antenna is replaced by a beam in which one dimension is narrow while the other dimension is broad such as a fan-shaped pattern.
One method of generating a fan beam is with a parabolic reflector shaped to yield the proper ratio between the azimuth and elevation beamwidths. Many long range ground based search radar antennas use a fan beam pattern that is narrow in azimuth and broad in elevation. When ground based search radar antennas employing fan beams are used against aircraft targets, no resolution in elevation is obtained. Therefore, no height information is available.
One method of achieving elevation angle information for targets located by a fan beam search radar antenna is to employ an additional fan beam radar antenna with the narrow dimension in elevation instead of azimuth, as in the common height finding radar antenna. In this method, again the height finding radar antenna actually measures elevation angle rather than height. Because the number of locations that the fan beam radar antenna must search is considerably less than the number that the pencil beam radar antenna must search, the fan beam radar antenna can dwell longer in each location so that more return signals per target can be obtained. The rate at which a fan beam antenna may be scanned is a compromise between the rate at which target position information is desired (data rate) and the ability to detect weak targets (probably of detection). Note that the two are at odds with one another and the more slowly the radar antenna scans, the more pulses will be available for integration and the better the detection capability. On the other hand, a slow scan rate means a longer time between the detection of the same target.
The simple fan beam antenna is usually inadequate for targets at high altitudes close to the radar antenna, because the fan beam antenna radiates very little energy in the high altitude direction close to the radar antenna. It is possible to modify the antenna pattern to radiate more energy at higher angles. One such technique for accomplishing high angle detection employs an antenna fan beam with a shape proportional to the square of the cosecant of the elevation angle.
In the cosecant-squared antenna, the gain is a function of the elevation angle and it should be noted that cosecant squared antennas apply to airborne search radar antennas observing ground targets as well as ground base radar antennas observing airborne targets. The cosecant-squared antenna may be generated by a distorted section of a parabola or by a true parabola with a properly designed set of multiple feed horns. The pattern may also be generated with an array-type antenna.
The cosecant-squared antenna has the important property that the echo power received from a target of constant cross-section at constant altitude is independent of the targets range from the radar. In theory, the mathematics illustrate that the echo power is independent of the range for the constant altitude target. However in practice, the power received from an antenna with a cosecant-squared pattern is not truely independent of range because of the simplifying assumptions made. It should be noted, that the crosssection of the target varies with the viewing aspect, the earth is not flat, and the radiation pattern of any real antenna can be made to only approximate the desired cosecant-squared pattern. For preliminary design purposes, it may be assumed that a search radar antenna having a pattern proportional to csc.sup.2 .phi., where .phi. is the elevation angle, produces a constant echo-signal power for a target flying at constant altitude if certain assumptions are satisfied. Fan beam search radar antennas generally employ this type of pattern.
The design of a cosecant-squared antenna pattern is an application of synthesis techniques which are generally found in the prior art literature. The cosecant-squared pattern may be approximated with a reflector antenna by shaping the surface or by using more than one feed horn. The pattern produced in this manner may not be as accurate as might be produced by a well-designed antenna array, but operationally, it is not necessary to approximate the cosecant-squared pattern very percisely. A common method of producing the cosecant-squared pattern employs a shaped reflector. The upper half of the reflector is a parabola and reflects energy from the feed horn in a direction parallel to the axis as is known in the art. The lower half of the reflector, however, is distorted from the parabolic contour so as to direct a portion of the energy in the upward direction.
A cosecant-squared antenna pattern can also be produced by feeding the parabola reflector with two or more feed horns or alternatively, by employing a linear array. If the feed horns are separated and fed properly, the combination of the secondary beams will give a smooth cosecant-squared pattern over some range of angle. A reasonable approximation to the cosecant-squared pattern has been obtained by employing two feed horns while a single feed horn combined with a properly located ground plane has been utilized to generate the pattern.
An example of a height finding system of the past that employed a pencil beam antenna was comprised of a rotator-type antenna or an array-type antenna each of which provided focus to the antenna beam. In the rotator-type antenna, the pencil beam is scanned in elevation as it is rotated. The angle of the pencil beam at the instant the signal is returned is labelled the elevation angle which is a necessary element for determining the height of the tracked target. Once the return beam is received, the elevation angle is measured for calculating the height of the tracked target.
There are many applications in which a knowledge of target height may not be necessary. An obvious example is where the target is known to lie on the surface of the earth, and its position is determined by range and azimuth. However, there are many instances in which a knowledge of the target's position in three dimensions is essential. The elevation angle can be used as the third position coordinate, but it is often more convenient to use height. Height may be derived from the measurement of range and elevation angle. The use of height, instead of the elevation angle from which it is derived, is more desirable in those applications where it is apt to be less variant than the elevation angle. This is usually true for aircraft targets or for satellites with nearly circular orbits.
Three-dimensional position information can be obtained with a symmetrical pencil-beam antenna. Both the azimuth and the elevation angle can be determined from a single observation with a single radar antenna. The pencil beam might search a hemispherical volume in space by rapidly nodding in elevation and slowly rotating in azimuth, or alternatively, the beam could rotate in azimuth while elevating slowly to trace out a helical-scan pattern. The chief disadvantage of a radar antenna with a pencil beam is that it usually requires a relatively long time to cover the volume of interest. The search time depends on the number of hits to be obtained from each target. The greater the number of hits per scan, the more accurate will be the angle measurement. The time t.sub.s required to scan an antenna of azimuth beamwidth .theta..sub.B and elevation beamwidth .phi..sub.B over a total azimuth angle .theta..sub.t and a total elevation angle .phi..sub.t when "n" pulses are to be received from each f resolution cell (with a pulse repetition frequency f.sub.r) is ##EQU1##
Consider a 2.degree. pencil beam that is to search a volume 360.degree. in azimuth and 60.degree. in elevation at a pulse repetition frequency of 1,000 Hz. If the scanning fluctuations are to be attenuated by 30 dB, at least 38 pulses must be processed per angular resolution cell. Substituting these values into equation (1) results in a frame time of 4.05 minutes. A 600-knot aircraft could fly 40.5 nautical miles in this time, which is a relatively long distance between observations. If three hits per scan were satisfactory, the frame time would be 0.27 minutes and the same aircraft would travel 2.7 nautical miles between observations. The pencil beam will generally be directed at targets above the ground clutter.
The rotation of the pencil beam in azimuth may be mechanical, as in conventional ground-based search radar systems. A rapid nodding scan is often used in elevation and may also be performed mechanically by moving the entire antenna. Alternatively, the parabolic torus with an organ-pipe scanner, or the planar array, or a linear array feeding a parabolic cylinder might also be used to scan the beam. The linear array could be electronically scanned in elevation and mechanically scanned in azimuth. Frequency scanning is a convenient form of electronic scanning for this application if the necessary bandwidth is available.
Elevation information may be obtained by stacking a number of narrow pencil beams in elevation and noting which beam contains the echo. Each of the stacked beams feeds an independent receiver. A separate transmitter might be used for each beam, or alternatively, a separate broad-coverage transmitting beam could illuminate the volume common to all narrow receiving beams. The overlapping pencil beams may be generated with a single reflector antenna fed by a number of horns--one for each beam. The beams may also be generated with an array antenna whose elements are combined to form a number of overlapping beams. By interpolating the voltages between adjacent beams of the stacked-beam configuration, it is possible to obtain a more precise measurement of the elevation angle than can be obtained with a single stationary pencil beam.
In many radar applications the fan beam is used to search the required volume. Even though the broad beamwidth of the fan beam in elevation does not permit the measurement of the elevation angle to any degree of precision, it is possible, in some cases, to obtain a rough approximation of target height. One technique makes use of the phenomenon whereby, under cetain circumstances, the pattern of a broad fan beam is broken into many smaller lobes by interference between the direct wave and the wave reflected from the surface of the earth. "Lobing" is more likely to occur at the lower radar frequencies and when the beam is located over water or other good reflecting surfaces. If the interference lobe pattern of the antenna is known--either by calculation or by calibration, using a known aircraft target--the range at which a target is first detected by the radar antenna is a measure of target height. The path of the target can be followed through the lobe pattern to obtain confirmation of the height. This technique is not too satisfactory since it offers but a crude estimate of height; it is not too reliable; it depends upon too many uncontrollable factors such as the propagation conditions; and it requires as a priori knowledge of the radar cross section of the target.
Another technique for measuring elevation angle or height includes the use of two antennas mounted one above the other. The elevation angle is measured by comparing the phase differences in the antennas as in an interferometer. Elevation angle can also be measured by generating two overlapping elevation fan beams with a single reflector as in the amplitude-comparison monopulse radar. The sum and difference signals are used just as in the monopulse tracking radar, except that the angle-error voltage does not control a servo loop but is used directly as a measure of the elevation angle.
The usual method of obtaining both azimuth-and elevation-angle measurements involves two separate fan-beam radar antennas. One of the two radar antennas is a vertical fan beam--narrow beam in azimuth angle, broad in elevation angle--rotating in azimuth to measure the range and azimuth. This is the conventional search radar antenna. A separate radar with a horizontal fan beam--narrow in elevation angle, broad in azimuth angle--is used to measure elevation. This is called a height finder. The range and azimuth obtained with the search radar antenna can be used to position the height finder in azimuth. The height finder searches for the target by scanning in elevation. Upon acquiring a target at the same range as indicated by the search radar antenna, it proceeds to nod about the target at a rapid rate to accurately determine the center of the beam. The search radar antenna and the height-finder radar antenna may be operated at two separate locations, or they may be mounted back-to-back on the same pedestal.
Another height-finding technique employed in the past is the V-beam radar antenna. This consists of two fan beams, one vertical and the other tilted at some angle to the vertical. The separation between the vertical beam and the slant beam may be 45.degree.. The time between observations of the same target in the two beams depends upon the target range and height. It can be shown that the height "h" of a target at a range R is ##EQU2## where .DELTA..omega.=azimuth rotation between beams=.omega..sub.s t.sub.h
.omega..sub.s =azimuth rotation rate, rps, and PA1 t.sub.h =time between observations, sec
Although an angle of 45.degree. may exist between the vertical and the slant beam, there is some advantage in making the angle smaller if the radar must operate with a high traffic density. The larger the number of targets, the more difficult is the problem of correlating the echoes from the two beams. The closer the beams, the easier it is to correlate the echoes.
As the beam is scanned in elevation, the elevation angle may be measured directly by a mechanical system. Since the pencil beam is always orthogonal to the radar antenna transmitter, the elevation angle can be measured as with a protractor-type measuring device assembled directly to the rotator-type antenna or with a synchro- servo measuring system. However, in a phased array radar antenna system, the pencil beam may be scanned in elevation by electronic phase shifters for achieving a rapid scan rate and for measuring the elevation angle. This type of system is employed when the data rate is important in tracking and is usually dependent upon the dynamics of the target. The main problem associated with the phased array pencil beam radar antenna is that in order to determine the target height by state-of-the-art methods, the system becomes very expensive to develop.
Certain radar systems of the past are based on a comparison of the amplitudes of echo signals received from two or more antenna positions. Some systems such as the sequential-lobing and the conical-scan techinques use a single, time-shared antenna beam while other monopulse techniques use two or more simultaneous beams. The difference in amplitudes in the several antenna positions is proportional to the angular error. The elevation angle may also be determined by comparing the phase difference between the signals of two separate antennas. Unlike the antennas of amplitude-comparison trackers, the antennas employed in phase-comparison systems are not offset from the axis. The individual boresight axes of the antennas are parallel, causing the radiation to illuminate the same volume of space. The amplitudes of the target echo signals are essentially the same from each antenna beam but the phases are different.
The measurement of the elevation angle by comparison of the phase relationships of the signals from the separated antennas of a radio interferometer is well known in the art and has been used as a passive instrument with the source of the energy being radiated by the target itself. A tracking radar antenna which operates with phase information is similar to an active interferometer and has been referred to as a phase-comparison monopulse or interferometer radar antenna.
Two receiving antennas are employed which are separated by a distance "d". Mathematical concepts have been derived for determining the electrical phase angle between the feedhorns. The electrical phase angle is a function of the distance "d", the elevation angle and the wavelength of the received energy. It should be noted that for an antenna operating on the phase-comparison monopulse or interferometer radar antenna principles, the distance "d" is limited. Thus, for the interferometer radar antenna to be able to substantially monitor the total hemisphere, then the distance "d" must be less than or equal to one-half the wavelength of the received energy to avoid elevation angle ambiguities.
In the general case, the frequencies of interest are those from the X-band to the L-band. Thus, if the distance "d" is to satisfy the above constraint, then the two antennas must be small. This is because the sum of the distance between the center lines of the two antennas must be equal to or less than one-half the wavelength of the received energy to avoid elevation angle ambiguities. Since the wavelength of frequencies of the L-band are approximately two feet while the wavelengths of the frequencies of the X-band are in the range of from one inch to two inches, the two antennas of the interferometer radar must be small. Generally, the radar range equation dictates that the bigger the antenna, the greater the gain or electronic amplification of the received signal. The radar equation generally describes the power of the received return signal in terms of the power transmitted from the radar antenna, the gain at the transmitting antenna, the gain at the receiver antenna, the wavelength of the received energy, the radar cross-section and the range from the radar antenna to the target.
A major problem associated with the interferometer radar is that the receive aperture is too small and cannot adequately receive the signal reflected from the target. Thus, if the receive aperture is too small, the antenna receiver is generally ineffective for long distance reception. The pair of antennas could be made larger to boost the gain by using a parabolic reflector. Under these conditions, the distance "d" becomes larger through the main lobe region and the effective elevation angle is depressed resulting in a small elevation coverage. Consequently, the interferometer radar antenna can only measure the elevation of a target within a limited specified range. If the target were above a particular elevation angle, the radar antenna would not detect it.
Therefore, in the interferometer radar antenna, two potential conditions exist. The first condition is when the distance "d" is within a specified length providing a larger elevation coverage but a low gain of the received signal. The second condition exists when the two antennas are made larger resulting in a higher gain of the received signals but a small elevation coverage.
Hence, those concerned with the development and use of height finding dual beam antennas in the radar field have long recognized the need for improved radar antenna tracking systems which provide a shaped reflector combined with a closely spaced pair of feedhorns for including a greater elevation angle coverage than if the reflector was shaped as a standard parabola and for providing more amplification gain than if the feedhorns had been the only receiving elements while simultaneously eliminating elevation angle ambiguities. Further, the shaped reflector of the improved radar antenna system should achieve a cosecant-squared antenna pattern, permit the determination of the three-dimensional position of the target by the addition of the azimuth angle, and be economical to manufacture in comparison with prior systems and methods of height finding.